1. Field of the Invention
This invention relates to circuitry for characterizing audio signals at the time they are being recorded on a recording medium for later reproduction. Data signals obtained from the characterizing information are also recorded on the recording medium which are usable at a later time to determine whether the recorded audio has been altered. In particular, the invention relates to such audio characterizing circuitry wherein the audio is characterized by the use of zero crossing detection circuitry.
There are situations where it is desirable to make a recording of audio information, such as speech or speech plus other audio, which will be played back at a later time with such recording made in a manner such that the authenticity of the recording, if questioned, can be verified electronically using a reasonable amount of electronic circuitry. Recordings made in such a manner have application in fields such as law enforcement, private investigation, journalism, medical, legal and many other similar fields where the authenticity of audio recordings might be questioned at some time.
2. Description of the Prior Art
A tamper protected recording system has been disclosed wherein input audio signals are recorded together with a first data signal that includes characterizing information derived from the input audio signals with a second data signal produced during playback that includes information of the same type obtained from input audio signals, with such second data signal derived from the recorded input audio signals. The two data signals are compared electronically. If the two data signals fail to provide a high level of comparison, the recording is considered to have been altered.
The level of comparison that can be attained with such a system using a reasonable amount of circuitry is directly related to the information contained in the input audio signals that is used to provide the data signal and the manner in which such information is obtained. Use of the envelope information presented by the input audio signal for characterizing information, for example, is not desirable since it does not contain sufficient information to permit the attainment of a desired level of comparison of the two data signals by the use of a reasonable amount of circuitry to provide an economically acceptable system.